1) Field of the Invention
The present invention relates to an automatic gain control circuit (which will be referred to hereinafter as an xe2x80x9cAGC circuitxe2x80x9d) suitable for use as an amplifying means for an optical receiver, and more particularly to an AGC circuit designed to convert an amplitude varying input signal into a signal with a constant amplitude.
2) Description of the Related Art
For example, Japanese Unexamined Patent Publication (Hei) No. 8-172332 discloses an AGC circuit for use in an optical receiver, where a conversion of an amplitude varying optical input signal into a signal with a constant amplitude takes place by means of the feedback of an amplitude control signal generated on the basis of an amplitude value of an output signal. FIG. 9 is a block diagram showing an example of a configuration of a conventional AGC circuit. This AGC circuit is made up of a variable gain amplifying means 41 for amplifying an input signal through gain variations to send out the amplified signal as an output signal, an amplitude detecting means 43 for detecting an amplitude value of the input signal, and a gain control means 42 for amplitude-controlling the variable gain amplifying means 41 on the basis of the detected amplitude value.
A control method for use in this AGC circuit is such that, when the amplitude detecting means 43 detects an amplitude value of an amplitude varying signal inputted to the variable gain amplifying means 41, the gain control means 42 generates an amplitude control signal on the basis of the detected amplitude value and feeds back the generated amplitude control signal to the variable gain amplifying means 41 for accomplishing the amplitude control. This enables converting an input signal varying in amplitude into a constant amplitude signal and outputting it.
There is a problem which arises with the above-mentioned conventional AGC circuit, however, in that, when a signal with a great amplitude is inputted to the variable gain amplifying means 41, the gain control means 42 generates a large amplitude control signal to feedback-control the variable gain amplifying means 41 so that there is the tendency for the variable gain amplifying means 41 to implement gain variable control beyond intention. Accordingly, the amplitude value of an output signal can vary in conjunction with amplitude variations of an input signal within a gain variable range of the variable gain amplifying means 41. In addition, the technique disclosed in the aforesaid Japanese Unexamined Patent Publication (Hei) No. 8-172332 is made to feedback control an amplitude value of an output signal of the variable gain amplifying means 41, and the input of a large amplitude signal likewise causes variation in output signal amplitude value.
The present invention has been developed with a view to eliminating such a problem, and it is therefore an object of the invention to provide an AGC circuit capable of preventing an amplitude value of an output signal from varying in conjunction with amplitude variations of an input signal within a gain variable range of a variable gain amplifying means even at input of a signal with a great amplitude, and an optical receiver using this AGC circuit.
For this purpose, in accordance with an aspect of the present invention, there is provided an AGC circuit comprising variable gain amplifying means for amplifying an input signal through the use of variable gain control to send out an output signal and gain control means for amplitude-controlling the input signal to the variable gain amplifying means, wherein the variable gain amplifying means includes peak detecting means for detecting a peak voltage value of the output signal, and the gain control means receives the peak voltage value detected by the peak detecting means to generate an amplitude control signal corresponding to an amplitude value of the output signal for feeding back the generated amplitude control signal to the variable gain amplifying means so that the variable gain amplifying means controls the amplitude of the output signal to a constant value in response to or with respect to amplitude variations of the input signal.
That is, according to the present invention, the AGC circuit comprises the peak detecting means for detecting a peak voltage value of an output signal of the variable gain amplifying means so that the gain control means generates an amplitude control signal corresponding to an amplitude value of an output signal through the use of the peak voltage value detected by the peak detecting means to feed it back to the variable gain amplifying means, thereby controlling the gain of the variable gain amplifying means. With this control method, it is possible to increase a variation of the peak voltage value corresponding to the amplitude value of an output signal with respect to amplitude variations of an input signal to the variable gain amplifying means. Therefore, owing to easy execution of control, even if a largely amplitude-varying input signal appears, the gain control means can control the amplitude of the output signal readily to a constant value.
In addition, in the AGC circuit according to the invention, the variable gain amplifying means serves as a differential amplifying circuit, and the peak detecting means detects a peak voltage value of one of two output signals of the differential amplifying circuit.
That is, with the AGC circuit according to the invention, the gain control means utilizes a peak voltage value of one of two output signals a differential amplifier constituting the variable gain amplifying means to generate an amplitude control signal corresponding to an amplitude value of the output signal on the basis of the peak voltage value so that the generated amplitude control signal is fed back to the variable gain amplifying means for controlling the gain of the variable gain amplifying means. This can control the amplitude value of an output signal of the variable gain amplifying means to a constant value in response to an amplitude varying input signal.
Still additionally, in the AGC circuit according to the invention, the variable gain amplifying means acts as a differential amplifying circuit and the peak detecting means individually detects peak voltage values of two output signals of the differential amplifying circuit so that the gain control means acquires the two detected peak voltage values to produce an amplitude control signal corresponding to an average value of the amplitude values of the two output signals.
That is, in the AGC circuit according to the invention, the peak detecting means individually detects the peak voltage values of the two output signals of the differential amplifier constituting the variable gain amplifying means. The gain control means generates an amplitude control signal corresponding to the amplitude values of the output signals on the basis of the average value of the two peak voltage values to feed it back to the variable gain amplifying means for controlling the gain of the variable gain amplifying means. This can control the output signals of the variable gain amplifying means to constant amplitude values equal to each other.
Furthermore, in accordance with another aspect of the present invention, there is provided an AGC circuit comprising variable gain amplifying means for amplifying an input signal through the use of variable gain control to send out an output signal and gain control means for amplitude-controlling the input signal to the variable gain amplifying means, wherein the variable gain amplifying means includes peak detecting means for detecting a peak voltage value of an output signal and direct-current voltage detecting means for detecting a direct-current voltage value of the output signal while the gain control means generates an amplitude control signal corresponding to an amplitude of the output signal on the basis of a difference between the detected peak voltage value and the detected direct-current voltage value to feed back the generated amplitude control signal to the variable gain amplifying means for controlling the variable gain amplifying means so that the amplitude of the output signal becomes a constant value with respect to amplitude variations of the input signal.
That is, in the AGC circuit according to the invention, the gain control means extracts a peak voltage value detected by the peak detecting means and a direct-current voltage value detected by the direct-current voltage detecting means to generate an amplitude control signal corresponding to an amplitude value of an output signal on the basis of a difference therebetween for feeding back the generated amplitude control signal to the variable gain amplifying means, thereby controlling the gain of the variable gain amplifying means. This can control the amplitude of an output signal of the variable gain amplifying means to a constant value with respect to an amplitude varying input signal and operating temperature variations in the variable gain amplifying means.
Moreover, in the AGC. circuit according to the invention, the variable gain amplifying means acts as a differential amplifying circuit, and the peak detecting means detects a peak voltage value of one of two output signals of the differential amplifying circuit, while the direct-current voltage detecting means detects a direct-current voltage value of the output signal from which the peak detecting means detects the peak voltage value.
That is, in the AGC circuit according to the invention, the gain control means generates an amplitude control signal corresponding to an amplitude value of an output signal on the basis of a difference between a peak voltage value and output direct-current voltage value of the output signal of the variable gain amplifying means to feed it back to the variable gain amplifying means for controlling the gain of the variable gain amplifying means. This can control the amplitude of an output signal of the variable gain amplifying means to a constant value in response to an amplitude varying input signal and operating temperature variations in the variable gain amplifying means.
Still moreover, in the AGC circuit according to the invention, the variable gain amplifying means acts as a differential amplifying circuit, and the peak detecting means separately detects the peak voltage values of two output signals of the differential amplifying circuit while the direct-current voltage detecting means separately detects the direct-current voltage values of the two output signals of the differential amplifying circuit so that the gain control means generates an amplitude control signal corresponding to an amplitude of the output signal on the basis of an average value of differences each between the peak voltage value and direct-current voltage value detected on each of the output signals.
That is, in the AGC circuit according to the invention, the gain control means generates an amplitude control signal corresponding to an amplitude value of an output signal on the basis of an average value of differences between the peak voltage values and direct-current voltage values of two output signals of the variable gain amplifying means to feed it back to the variable gain amplifying means, thereby controlling the gain of the variable gain amplifying means. This can control the amplitudes of the two output signals of the variable gain amplifying means to constant values equal to each other with respect to an amplitude varying input signal and operating temperature variations in the variable gain amplifying means.
Furthermore, in accordance with the present invention, there is provided an optical receiver using an AGC circuit stated above.